1. Technical Field
The present invention relates generally to an electromagnetic relay which may be installed in a motor circuit of san engine starter and equipped with a built-in resistor which controls a starting current in an electric motor when starting, for example, an internal combustion engine and supplies the current to the electric motor at a full voltage bypassing the resistor after the internal combustion engine is started up.
2. Background Art
Typical engine starters for use in starting an internal combustion engine mounted in, for example, an automotive vehicle are equipped with an electromagnetic switch which works to push a pinion to a ring gear of the engine and to close main contacts in a motor circuit to supply electric current from a storage battery to an electric motor installed in the engine starter.
When the electric motor is turned on, in other words, the electromagnetic switch closes the main contacts, it will cause an excessively large current called the inrush current to flow from the battery to the electric motor, so that the terminal voltage of the battery will drop greatly, which may result in an instantaneous failure in operation of electrical equipment such as indicators or an audio system installed in the vehicle which will also be referred to as a short break.
Japanese Patent First Publication No. 2009-224315, assigned to the same assignee as that of this application, discloses techniques to control the inrush current induced immediately following turning on of the electric motor. Specifically, the disclosed system is equipped with an electromagnetic relay separate from the electromagnetic switch installed in the engine starter. The electromagnetic relay works to open or close the motor circuit selectively and has installed therein a resistor connected electrically to the motor circuit and relay contacts disposed in parallel between an upstream end and a downstream end of the resistor. The electromagnetic relay is of a normally open type in which when a motor drive signal is in an off-state, that is, a relay coil is deenergized, the relay contacts are opened, while when the motor drive signal is in an on-state, that is, the relay coil is energized, the relay contacts are closed.
When it is required to start the engine, the electromagnetic switch installed in the starter closes the main contacts in the condition where the motor drive signal outputted to the electromagnetic relay is in the off-state, so that the relay contacts are opened. This causes the starting current, as controlled by the resistor, to be supplied to the electric motor, so that the electric motor starts to rotate at a low speed. After the pinion is brought into engagement with the ring gear of the engine, the motor drive signal is switched to the on-state, so that the relay contacts are closed to short-circuit the ends of the resistor, thereby supplying the power to the motor at a full voltage of the battery to rotate the motor at a high speed.
The electromagnetic relay is, as described above, of a normally open type which keeps the relay contacts opened when the motor drive signal is in the off-state. Therefore, if a vehicle system fails in operation due to breakage of a motor drive signal line or poor insertion thereof into an electrical connector, it will keep the relay contacts opened. The electromagnetic relay is, therefore, not energized in response to the motor drive signal outputted from an electronic control unit (ECU), so that the relay contacts are kept opened. This causes the current, as supplied from the battery, to flow to the electric motor through the resistor at all the time. For instance, in a cold condition where the outside temperature is low, a load on the electric motor is usually increased with an increase in mechanical friction of the engine, and a resistance value of the motor circuit is decreased, so that more current flows through the resistor.
Further, even when the electric motor is turned on, and the pinion engages the ring gear, the relay contacts are not closed, so that the voltage is not applied to the electric motor fully, thus resulting in a difficulty in starting the engine, that is, lowered start-up performance of the engine. The continuous flow of current to the electric motor through the resistor may cause the resistor to be melted down, which leads to impossibility to start the engine.